Reciprocable devices have application in both the generation and utilization of power. For example, certain motors and positive displacement pumps and compressors utilize a reciprocable member which may be, for example, a piston or diaphragm.
Fluid, i.e., liquid or gas, under pressure can be used to reciprocate the reciprocable member. One problem with using fluid under pressure to reciprocate a reciprocable member is getting the reciprocable member to properly reverse its direction at the end of each stroke and not stall. A related problem is being able to start the reciprocable member moving along one of its strokes regardless of the position of the reciprocable member at startup. In this regard; a reciprocable device may be shut off with the reciprocable member in any position along its path of travel, and accordingly, restarting of the reciprocable device must be possible regardless of the location of the reciprocable member.
To reciprocate a reciprocable member, fluid under pressure is alternately supplied to the opposite sides of the piston, and the side of the piston which is not subjected to fluid under pressure is vented or exhausted. These functions are typically accomplished by a switching mechanism which comprises a pressure valve for controlling the flow of fluid under pressure and exhaust valves for controlling the exhausting process.
Reciprocating devices are shown, for example, in Hartley et al U.S. Pat. No. 4,610,192. The patented construction employs a bistable toggle mechanism which is driven just over center by energy from the piston and then driven by stored spring energy. The toggle action reverses the pressure and exhaust valves to bring about a reversal of movement of the reciprocable member.
Although these patented constructions are most satisfactory for many applications, it has been found that, for high-pressure applications, a strong spring must be used to assure switching of the valves. This relatively high spring force holds the reciprocable member in either of two positions even when the device is not in use, and as a consequence, the seating surfaces of the valves tend to take an undesirable permanent set.